1. Field of the Invention
The present invention relates to an automatic transmission having a planetary gear unit especially suitable for application to an FF (i.e., Front-engine Front drive type) automobile and, more particularly, to a five-speed automatic transmission, which can be converted to a four-speed automatic transmission by a slight change.
2. Description of the Related Art
Generally speaking, a four-speed transmission combines an auxiliary transmission section having one planetary gear mechanism and a main transmission section having two planetary gear units to establish four forward speeds and one rear speed. The application of this type automatic transmission to a FF automobile is shown, for example, in our U.S. Pat. No. 4,722,242 which discloses a four-speed automatic transmission, which combines a three-speed automatic transmission mechanism, having two planetary gear units, and an under drive (UD) mechanism, having a single planetary gear unit.
In recent years demand for improvements in mileage and running performance has led to development of transmissions with five forward speeds. One such automatic transmission including, arranged on a first shaft, a main transmission mechanism, having a combination of a single planetary gear set and a dual planetary gear set, and arranged on a second shaft, a Simpson type auxiliary transmission mechanism, as disclosed in our prior pat U.S. Pat. No. 4,914,978. Thus, the main transmission mechanism can establish three or four forward speeds and is combined with a three-speed auxiliary transmission mechanism to establish five or six forward speeds for the automatic transmission as a whole (hereinafter the "multi-speed" automatic transmission A).
Many types of automatic transmissions are required to meet the needs of different vehicles and different preferences. It would lead to a drastic increase in production cost if such various kinds of automatic transmissions were to require correspondingly different parts and different assembly lines. Hence, it is an important consideration in the manufacture of automatic transmissions that the various parts and the various assembly lines should be interchangeable for different transmissions. The capability to modify a four-speed automatic transmission into a five-speed one is conventional.
In the aforementioned multi-speed automatic transmission A, the auxiliary transmission mechanism has its two planetary gear sets connected by a connecting member and has, at its rear (near the engine), a 2nd-speed brake and a 3rd-speed clutch. As a result, the number of parts which can be shared with the four-speed automatic transmission, equipped with the auxiliary transmission mechanism having a single planetary gear set, is so few that the brakes, clutches and so on have to be changed into different arrangements and connections, as between the four- and five-speed automatic transmissions, thus requiring drastic changes in assembly lines. Thus, the multi-speed automatic transmission A encounters the problem of high production cost.
When the automatic transmission has multiple speeds, moreover, a wider range of gear ratios can provide the advantage that a higher torque can be achieved at lower-speed gear stages to improve the acceleration of the vehicle. On the other hand, the frictional engagement elements which engage at the lower speeds are exposed to higher loads so that they require enhanced durability. Such frictional engagement elements are typically multi-disc engagement elements having higher torque capacities. However, the multi-disc engagement elements must have many frictional members and hydraulic servos acting axially. Thus, the multi-disc engagement elements cause an increase in axial dimension of the automatic transmission so that they cannot always be advantageously used in the FF automatic transmission which is limited with regard to permissible axial length.
In the auxiliary transmission mechanism of the aforementioned multi-speed automatic transmission A, band brakes are used as the frictional engagement elements to be engaged in lower-speed gear stages, e.g. for 1st speed and reverse, so as to minimize increase in the axial length. With a view to increasing the torque capacity, however, the band brakes are longer than conventional band brakes. Thus, the use of band brakes does not adequately serve the purpose for which they were adopted, i.e. to reduce dimensions of the transmission.
Therefore, there has been proposed a multi-speed automatic transmission (hereinafter "multi-speed automatic transmission B") having a Simpson type auxiliary transmission mechanism with multi-disc brakes, as disclosed in U.S. Pat. No. 4,624,154. In this multi-speed automatic transmission B, the frictional engagement elements which engage in the higher-speed gear stages with relatively lower load are band brakes, whereas the frictional engagement elements which engage at a lower speed gear stage under higher load are multi-disc brakes having higher torque capacities.
In the aforementioned multi-speed automatic transmission B, however, the auxiliary transmission mechanism must have an axial dimension sufficient to accommodate hydraulic servos for applying and releasing the multi-disc brakes, as described above. At the same time, the parts are not interchangeable with (shared in common with) a four-speed automatic transmission, e.g. the foregoing multi-speed automatic transmission A, because the two planetary gear sets are connected by the connecting member and have the multi-disc brakes and the direct drive clutches at the engine end. Therefore, there remain serious unsolved problems such as lack of a feasibly short axial length and remarkably high cost.